1. Field of the Invention
The invention pertains to nickel batteries.
2. Art Background
A nickel battery (for purposes of the invention) is a battery which includes a cell (or cells) where the electrochemically active material at the positive electrode of the cell contains nickel hydroxide (Ni(OH).sub.2). (Electrochemically active material is material which stores and releases electrical charge when a constituent of the material electrochemically undergoes, respectively, a valency increase and a valency decrease.) During charging, the Ni(OH).sub.2 is oxidized to form NiOOH (with the valency of Ni increasing from 2 to 3), while during discharging, the NiOOH is converted to Ni(OH).sub.2.
Included among the different types of nickel batteries are, for example, nickel-hydrogen (Ni-H.sub.2), nickel-cadmium (Ni-Cd), nickel-zinc (Ni-Zn), and nickel-iron (Ni-Fe) batteries. In a Ni-H.sub.2 battery, hydrogen (H.sub.2) gas is evolved at the negative electrode of the cell during charging. In a Ni-Cd, Ni-Zn, or Ni-Fe battery, the negative electrode includes Cd, Zn, or Fe, respectively (hence the nomenclature), when the battery is charged.
Nickel batteries have a number of advantageous properties including the fact that they are rechargeable (they can be charged and discharged more than 200 times) and have relatively high energy densities (stored charge per unit weight or per unit volume) compared, for example, to lead-acid batteries. In addition, nickel batteries are relatively inexpensive compared to noble metal batteries (which also exhibit high energy densities) such as silver-zinc (Ag-Zn) or silver-cadmium (Ag-Cd) batteries. These advantageous properties have resulted in nickel batteries being widely used in, for example, earth-orbiting satellites, toys, and battery powered shavers.
Shortly after manufacture, a nickel battery generally undergoes a characterization procedure to determine the capacity of the battery (the maximum amount of charge, typically expressed in Ampere-hours, which the battery will deliver during discharging). During this characterization procedure, the battery (which is initially discharged) is successively charged and discharged, usually 3 or 4 times. The first charging step generally produces a relatively large capacity increase, with each successive charging step (interrupted by intervening discharging steps) producing successively smaller increments in capacity. After the third or fourth charging step, no further increase in capacity is usually achieved. If the battery is defect-free, then the maximum capacity achieved using this characterization procedure will be equal, or approximately equal, to the theoretical capacity, C (the amount of charge which the battery is theoretically capable of storing if all of the Ni(OH).sub.2 is converted to NiOOH), of the battery. (Theoretical capacities of individual nickel battery cells used, for example, in earth-orbiting satellites are typically about 25 Ampere-hours.)
A significant problem associated with both used and unused (new) nickel batteries is that they suffer a substantial (as much as 50-60 percent) loss of capacity when stored for relatively long periods of time (a month or more) at room temperature, or when stored for relatively short periods of time (a week or less) at temperatures higher than room temperature. (Batteries are normally stored discharged and shorted, i.e., the battery terminals are electrically connected to one another, to prevent sparking and the possibility of fires.) That is, when nickel batteries are removed from storage and subjected to the usual characterization procedure, the measured capacities are substantially lower than their theoretical capacities (if, for example, C=25 Ampere-hours, then the measured capacities are typically 15 Ampere-hours). (During the characterization procedure, the first charging step produces a jump in capacity up to about 13 or 14 Ampere-hours, the second charging step produces a further capacity increase of about 0.5 Ampere-hours, and each successive charging step produces rapidly decreasing increments in capacity.) This problem of capacity loss is particularly prevalent in nickel batteries where the Ni(OH).sub.2 is electrochemically deposited (regarding electrochemical deposition of Ni(OH).sub.2 see, e.g., U.S. Pat. No. 3,653,967 issued to R. L. Beauchamp on Apr. 14, 1972).
Attempts to restore the lost capacities (lost during storage) of nickel batteries, i.e., to increase capacities beyond the levels achieved with the usual characterization procedures, have involved continuously charging the batteries at a constant charging rate (achieved by imposing a voltage between the battery terminals which yields a constant current). The charging rates (typically expressed as a fraction or a multiple of C per hour) have generally ranged from about C/20 per hour up to about C/10 per hour. In addition, the total charge, Q, delivered to the batteries has generally been much larger than C, with the ratio Q/C greater than 1.6. (The batteries were heavily overcharged because it was believed that this would induce a valency increase in nickel from 2 to 4 as well as from 2 to 3, and thus the batteries would achieve capacities greater than their theoretical capacities.) Charging rates greater than about C/10 per hour (with heavy overcharging) have been avoided because such high rates lead to the liberation of undesirably large amounts of O.sub.2 from within the electrolyte, a process which is exothermic. The resulting electrolyte temperature increase results in, for example, H.sub.2 (in a Ni-H.sub.2 battery) reacting with NiOOH to form Ni(OH).sub.2 at so great a rate as to reduce the capacity of the battery.
Despite the use of heavy overcharging, none of the continuous charging techniques has proved useful in fully recharging, or even significantly increasing the reduced capacity (reduced below the theoretical capacity) of, a nickel battery which has suffered a capacity loss (typically 10 Ampere-hours for a cell where C=25 Ampere-hours) during storage. For example, while a charging rate of C/10 per hour produced a larger increase in capacity than a charging rate of C/20 per hour, this increase in capacity was typically no more than about 0.02 Ampere-hours. Moreover, repeating the charging procedure, using the same charging rate used initially, or increasing the amount of overcharge, produced little or no increase in capacity beyond that achieved initially. However, repeating the charging procedure at a higher charging rate did produce a further increase in capacity. These results have led to the belief that the only method for systematically increasing the reduced capacity (reduced below its theoretical capacity) of a nickel battery is to repeatedly charge the battery, but at a charging rate which increases with each repetition. But the undesirably large electrolyte temperature increases associated with charging rates greater than C/10 per hour have precluded the possibility of using such a procedure to fully restore nickel batteries to their theoretical capacities, or to even significantly increase the reduced capacities of nickel batteries.
Thus, those engaged in the development of nickel batteries have sought, thus far without success, a method for recovering the lost capacities of nickel batteries without requiring alteration of the battery structure.